WASHINGTON, Jan 31 (Reuters) - Two senior U.S. Democratic lawmakers on Wednesday asked Facebook Inc and Twitter Inc to answer questions about potential Russian involvement in social media campaigns that urged release of a Republican memorandum alleging surveillance abuses at the FBI, saying the companies had provided insufficient responses so far.

The letter was the latest effort by Democrats, in the minority in both the U.S. Senate and House of Representatives, to press social media companies on whether the Kremlin has used their platforms to meddle in U.S. politics.

"It is unclear from your responses whether you believe any of the Russian-linked accounts involved in this influence campaign violated your respective user policies," Senator Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, and Representative Adam Schiff, the top Democrat on the House Intelligence Committee, said in a joint statement.

"We reiterate our request that you immediately take necessary steps to expose and deactivate such accounts if you determine that they violate your respective user policies," they added.

At issue is an online campaign, using the hashtag

#ReleaseTheMemo, aimed at ensuring the release of the controversial, classified memorandum written by Republicans on the House Intelligence Committee.

Democrats say the memo alleging surveillance abuses is highly misleading, based on a selective reading of highly classified material and designed to distract from special counsel Robert Mueller's investigation into potential collusion between Russia and President Donald Trump's 2016 campaign.

In its response, released publicly on Wednesday by the lawmakers, Twitter said a preliminary analysis using available geographical data related to the use of the #ReleaseTheMemo hashtag did not identify "any significant activity connected to Russia."

Twitter also said it observed use of the hashtag by high-profile accounts that have a high number of followers, which "plays a role in driving conversations around a hashtag."

It questioned the findings of an online dashboard maintained by the Alliance for Securing Democracy, a project of the nonpartisan German Marshall Fund think tank, because the group does not publicly disclose the accounts it tracks.

Earlier this month, the alliance found a huge spike in use of the hashtag by Twitter known or suspected to be under Kremlin influence. (Reporting by Dustin Volz; Editing by Bill Rigby and Bill Trott)